When and how did viruses evolve into their current form (i.e. basically a mobile DNA syringe)? I suspect they predate multicellular organisms, but how far back do they go?
Obviously, they couldn’t exist until there was a large supply of living cells around them. But given how seemingly universal antiviral defenses are (as in almost all beings have some type of defense, even if it varies widely in form and function from one thing to the next) they probably appeared pretty early.
Not necessarily. They couldn’t exist in today’s world without the living cells, but in the early days of the primordial sludge, the lines between living and nonliving were blurrier than they are today. It’s possible that the first entities which catalyzed their own replication were closer to what we would call viruses than to what we would call cells.
Maybe, in a sense, but I doubt whether that was where the viruses around today, or even those that first began to parasitize cellular life, way back when, came from. Much more likely, the first true viruses were essentially bits of “rogue” DNA or RNA that became separated from some cell’s genome.
I don’t know that it is possible to say much more than that, however. Viruses do not really fossilize. Indeed, unicellular life scarcely does, except sometimes when it forms large masses of cells, so there is rather little direct evidence about the first billion years or so of the evolution of life, and really none at all about viruses. All we can really do is speculate, based on what we know about how viruses behave now, and our relatively meager knowledge of what cellular life was like back then.
If they totally catalyzed their own replication, I’d say that, per definition, they weren’t viruses. As I understand it, a virus is an entity that parasitizes on the self-replicating machinery of an external agent.
As I understand it, most viruses today are bacteriophages, which rely solely on a unicellular host. Given that most of life as we know it today is unicellular, and that multicellular life probably has existed for less than half of life’s history on earth, I would bet that the first viruses exploited single-cell organisms.
They evolved early on in the history of life, and multiple times (as we can tell by molecular clocks, or so I’m told.).
There are three main strands of thought - that they were originally single-celled parasites of other single-celled organisms that became simplified, that they were bits of other cell’s genetic material that escaped control (like a mobile cancer for single cells, if you squint) or that they evolved at the same time or even before other cells. That last one is least likely, but all 3 ideas have problems. I favour the first theory, myself.
I’m sure that soon after the arrival of organism Alpha, the first stretch of RNA that could catalyze its own replication, some other stretch of RNA arose that could reproduce only by using some intermediate molecules produced by Alpha. And that first virus was probably a snipped-down version of Alpha.
I was at a seminar recently by a guy who produced some great EM images showing pretty convincingly that the classic “spaceship” type bacteriophage - Bacteriophage - Wikipedia - bears some striking resemblance to a type of secretion system bacteria use to fight each other.
I think it’s quite widely accepted that viruses, as obligate parasites, evolved after their hosts. Things like transposons and secretion systems point to some reasonable and plausible mechanisms as to how they could have developed.
Obviously, they couldn’t exist until there was a large supply of living cells around them. /QUOTE]
Well the life form that is defined as a virus could not exist. But the proto-virus, the self-replicating predecessor, could exist.
Perhaps better to think that the cell walls are DNA/RNA defense against the other DNA/RNA… One life form evolved against having its patch invaded, and another evolved the ability to invade the others patch… and take over.
So life forms evolved cell walls… Along the way the proto-virus loses the ability to replicate itself… so becomes a virus.
Thanks for the info, all. The possibility of free-floating RNA bits never occurred to me, but it makes a lot of sense now that I think about it. Simplified predator is kind of like what I’d originally imagined.
